The invention relates to integrated circuit manufacturing technology, and more specifically, to processes for planarizing surfaces of wafer-type semiconductor substrates, such as semiconductor wafers, through chemical mechanical polishing.
Photolithographic optics-based processes are used in the manufacture of integrated circuits, and since these processes require accurate focusing to produce a precise image, surface planarity becomes an important issue. This is becoming increasingly critical as line widths are being reduced in size in order to make semiconductor devices even more compact, and to provide higher speeds. More accurate optical focusing for finer line widths results in a loss of xe2x80x9cdepth of fieldxe2x80x9d (i.e., the focusing is very accurate only in a plane of very limited depth). Accordingly, a planar surface is essential to ensure good focusing to enable the photolithographic process to produce fine line width, compact high speed semiconductor devices.
There are several techniques for planarizing the surface of a semiconductor wafer. One of these is chemical mechanical polishing (CMP). As indicated in an article entitled xe2x80x9cChemical Mechanical Polishing: The Future of Sub Half Micron Devices,xe2x80x9dDr. Linton Salmon, Brigham Young University (Nov. 15, 1996), CMP is now considered the most effective method yet for planarizing wafers with sub micron lines. In this process, a wafer is mounted on a rotary carrier or chuck with the integrated circuit side facing outward. A polishing pad is then brought into contact with the integrated circuit side. Pressure may be applied by the carrier and/or the platen to effectuate polishing. According to Salmon, in some CMP machines the wafer rotates while the polishing pad is stationary, in others the pad rotates while the wafer carrier is stationary, and in yet another type both the wafer carrier and the pad rotate simultaneously. The polishing pad may be pre-soaked and continually re-wet with a slurry that has a variety of abrasive particles suspended in a solution. Typically, the particles range in size from 30 to 1,100 nanometers. After planarization through polishing, the wafers go through a post-CMP clean up to remove residual slurry, metal particles, and other potential contaminants from its surface.
An important variable in planarization through CMP is xe2x80x9cremoval ratexe2x80x9d which is the rate of removal of material from the surface of the semiconductor wafer being polished. Preferably, the rate of removal should be such that any surface peaks are preferentially flattened and the resultant surface should be as near perfectly planar as possible. There are several factors that may affect the rate of removal. For example, the nature of the slurry can have a dramatic effect. The slurry includes abrasive particles suspended in a solvent which selectively may soften certain features of the pattern on the semiconductor wafer surface, thereby affecting the relative rate of removal of those features vis-à-vis others. As indicated in the above article, xe2x80x9cThe purpose of the slurry is simple, yet understanding and modeling all the mechanical and chemical reactions involved is nearly impossible.xe2x80x9d Accordingly, development of the CMP process has proceeded on a xe2x80x9ctrial and error basis.xe2x80x9d
Among the more advanced CMP machines presently available are the AvantGaard Model 776 of IPEC of Phoenix, Ariz. In this CMP apparatus, the lower head (containing the polishing pad) orbits, while the carrier holding the wafer rotates about a central axis. Polishing fluids (slurry) are introduced to the wafer directly through the polish pads with point-of-use mix, which results in better wafer uniformity and reduced slurry consumption.
There continues to be multiple challenges in CMP, making the polishing and planarization faster, more uniform across a wafer, and improving the variation seen in wafer to wafer results. The polishing motion of the pad and carrier play a crucial role in the CMP process along with the quality of the polishing pad over its life.
The polishing pad should be xe2x80x9cconditionedxe2x80x9d after a period of use to provide for a more uniform polishing rate, from wafer to wafer, and to provide for better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner arm with an abrasive lower surface is forced to come in contact with the pad upper surface while the pad oscillates and the conditioner arm moves back and forth in an arc about a pivot axis outside of the circumference of the polish pad. The combined pad oscillation and the conditioning arm arc motion during conditioning results in non-uniform pad surface removal and roughing. Areas closer to the arm arc pivot are conditioned at a higher rate than the areas more distant from the arc pivot. Over time, this non-uniform pad conditioning results in poorer polishing uniformity on the semiconductor wafers.
Semiconductor manufacturers consistently require CMP processes to improve over time. As semiconductor devices become ever more complex and device geometry becomes ever so much smaller, there exists a need to make the CMP removal rate more consistent from wafer to wafer and wafer lot to wafer lot, while also making the polishing results more uniform across the entire surface of a wafer. Furthermore, there is also a need for a method to provide better and more uniform conditioning of CMP pads during their lifetime.
The invention provides a method of improving the uniformity of the rate of removal of material from the surface of a semiconductor substrate, such as a wafer having integrated circuits formed thereon. The invention also provides a method for better and more uniform conditioning of chemical mechanical polishing (CMP) pads to extend the useful lifetime.
The objective of the invention is achieved through use of a combination of polishing motions applied to the surface of the semiconductor substrate or cleaning motions applied to the polishing pad. These motions are selected from combinations of the following: rotational, orbital, oscillating, sweeping and linear movement. As explained in more detail herein, the combination of motions may be achieved through permutations of movement of the polishing platen and wafer carrier, in the case of semiconductor substrate surface polishing; and through permutations of the movement of the polishing pad and conditioning surface, during polishing pad conditioning.
In accordance with one embodiment of the method of the invention, a wafer held in a carrier (which may rotate about a central axis, or which may be stationary) is brought into contact with a polishing pad that is rotating or oscillating (i.e., at least partially rotating in alternating directions) about its central axis, while the pad is simultaneously orbiting around an orbital axis. The clockwise and counter-clockwise rotational oscillations of the polishing pad about its central axis may range through angles of less than 360 degrees to more than 360 degrees in each direction. Continuous rotation of the polishing pad about its central axis may also be imparted in certain embodiments to improve the surface characteristics of the semiconductor wafer. The wafer carrier may be rotated or oscillated about an axis or held stationary. A polishing slurry is applied, either through the pad itself or through distribution onto the pad to allow infiltration between the pad and the wafer surface being polished. The polishing is maintained while applying a sufficient pressure to polish the semiconductor wafer surface to a desired degree of planarity.
In accordance with another embodiment of the method of the invention, a wafer held in a carrier is brought into contact with a polishing pad that is moving linearly, relative to the wafer surface. The wafer carrier on the other hand both orbits about an axis, and oscillates about a second axis, offset from the first axis. Alternatively, the polishing pad may rotate about a central axis.
The current embodiment of the invention also provides an apparatus for polishing semiconductor wafers to planarize the surfaces of the wafers. The apparatus includes a carrier adapted for securely holding at least one semiconductor wafer to expose the back surface of the wafer to be polished on an underside of the carrier, and the front surface of the wafer to be polished to a polishing pad, supported on a platen, spaced from the carrier underside. But one with ordinary skill in the art could orient the apparatus such that the carrier was below the platen. The apparatus includes mechanical means for imparting orbital motion to the platen. Such means, for example, may include a stacked pair of rotary bearings with an upper bearing fixedly mounted to the platen and an upper portion of a cylindrical sleeve, that has central axes in its upper and lower portions offset from each other, extending vertically below the platen. A lower bearing is mounted to the lower portion of the cylindrical sleeve and housing of the apparatus, such that the axes of rotation of the bearings are offset. A drive motor rotates the sleeve, thereby causing the platen to orbit about an orbital axis. The apparatus of the invention further includes a shaft having a first end coupled to the platen supporting the polishing pad, and a second end coupled to means for imparting rotating or oscillating motion to the shaft. These means may include, for example, a drive motor with a gear box to rotate the shaft and a motor controller to control degrees of rotational output of the motor. Alternatively, a mechanical stop means may limit the arc of rotation of the shaft, and an electrical stop may reverse oscillatory motion of the shaft when the stop has been reached. Other mechanical devices for controlling degrees of shaft rotation or oscillation are clearly also useful. The wafer carrier may be rotated or oscillated about its axis by suitable means, or remain stationary.
The invention also provides an apparatus in which the wafer carrier undergoes orbital and rotational motion, or orbital and oscillating motion; while the pad in contact with the semiconductor substrate held in the carrier either rotates, or is held stationary. In accordance with this apparatus, the mechanical means for imparting orbital and rotational or oscillating movement to the carrier substantially corresponds to the above-described apparatus for imparting such motion to the platen. The platen holding the polishing pad, in accordance with this embodiment, has a central shaft that may be rotated at a controlled rate by an electrical motor, or maintained stationary. Thus, a substrate held in the wafer carrier has a surface subjected to potentially one of four types of permutations of polishing motion: (1) orbital and rotational (with platen stationary); (2) orbital and rotational and sweeping (with platen rotating); (3) orbital and oscillating (with platen stationary); and (4) orbital, oscillating, and sweeping (with platen rotating).
In a yet further embodiment of the invention, the pad is a continuous belt mounted over a pair of rollers, and has a backing slide plate to allow pressing of the belt pad against a semiconductor substrate held in a wafer carrier. In this embodiment, the wafer carrier is able to produce orbital motion, and either oscillating or rotational motion. Thus, when the continuous belt is driven linearly, the surface of the semiconductor substrate is subjected to one of the following two polishing motions: (1) a combination of orbital oscillation and linear movement; and (2) a combination of orbital, rotational and linear polishing movement.
Using the apparatus of the invention, and applying the method of the invention, semiconductor wafers are produced that are more planar across the entire surface area, than wafers that are polished without the rotary or oscillatory motion of the invention. The removal rate of the method and apparatus of the invention is more uniform across the wafer.
The invention also provides, through the at least partial rotational movement and simultaneous orbital movement of the pad, a method for improving pad conditioning. Usually, as explained before, in the prior art pad conditioning process, a pad conditioner arm with an abrasive lower surface is brought into contact with the pad upper surface while the pad oscillates and the conditioner arm moves back and forth in an arc about a pivot axis outside of the circumference of the polish pad. The combined pad oscillation and the conditioning arm motion during conditioning results in non-uniform pad surface removal and roughing. Over time, this non-uniform pad conditioning results in poorer polishing uniformity on the semiconductor wafers. In accordance with the invention, the rotation or oscillation of the pad greatly enhances the conditioning process by allowing the areas that ordinarily are less conditioned in the prior art to move into regions of higher conditioning while the more heavily conditioned areas move into the regions of lower conditioning. Thus, uniform conditioning across the pad may be achieved through the invention.